Activity of simian DNA-binding factors is altered in the presence of simian virus 40 (SV40) early proteins: characterization of factors binding to elements involved in activation of the SV40 late promoter.

The early proteins of simian virus 40 (SV40) large T and small t antigen (T/t antigen) can each cause the transcriptional activation of a variety of cellular and viral promoters. We showed previously that simian cellular DNA-binding factors (the Band A factors) bind to sequences within the SV40 late promoter which are important for transcriptional activation in the presence of the SV40 early proteins. Band A factors isolated from simian cells which produce T/t antigen (COS cells or SV40-infected CV-1 cells) have altered binding properties in comparison with the factors from normal simian cells (CV-1). This suggests that the transcriptional activation mediated by T/t antigen may be due to either modification of existing factors or induction of new members of a family of factors. We have purified the Band A factors from both COS and CV-1 cells and have determined the binding site by methylation interference and DNase protection footprinting. The COS cell factors have altered chromatographic properties on ion-exchange columns and have higher-molecular-weight forms than the CV-1 cell factors. Major forms of the CV-1 factors migrate between 20 and 24 kilodaltons, while the COS factors migrate between 20 and 28 kilodaltons. The binding sites for the factors from CV-1 and COS cells are similar, covering a rather broad region within the 72-base-pair repeat comprising the AP-1 site and the two-octamer binding protein (OBP100/Oct 1) sites, OBP I and OBP II. Specific binding competition analyses indicate that the two general regions within the binding site (the AP-1-OBP II site and the OBP I site) each retain partial binding ability; however, the factors bind best when the two regions are adjacent in a relatively specific spatial arrangement. The binding site for the Band A factors corresponds very well to sequences necessary for the activation of the late promoter as defined by deletion and base substitution mutagenesis studies (J. M. Keller and J. C. Alwine, Mol. Cell. Biol. 5:1859-1869, 1985; E. May, F. Omilli, M. Emoult-Lange, M. Zenke, and P. Chambon, Nucleic Acids Res. 15:2445-2461, 1987). These data, in combination with the data showing that the Band A factors are modified or induced in the presence of T/t antigen, strongly suggest that T/t antigen mediates its transcriptional activation function, at least in part, through the Band A factors.